In many fields including electric home appliances, vehicles, construction materials, etc., it is common to subject a steel sheet or a surface-treated steel sheet to a chromate treatment in order to give it anti-rust or paint adhesion to an upper layer. However, since the chromate-treated steel sheet typically contains hexavalent chrome which is noxious to environments, need for hexavalent chrome-free surface-treated steel sheets has been recently increasing, and some related industries are trying to do away with steel sheets containing hexavalent chrome.
Under such circumstances, various methods for treating a surface of a steel sheet without using chromes have been proposed. For example, for a film containing an organic compound as a main component, Patent Document 1 discloses a method of treating the film with a treatment agent containing a phosphoric acid, an aluminum sol and a metal hydrosol, Patent Document 2 discloses a method of treating the film with water glass or sodium silicate and pyrazole, and Patent Document 3 discloses a method of coating the film with silicate. However, the organic film has a problem in that marks are likely to occur in machining and shaping the film and adhesion of the film to an overcoating paint is low, thereby limiting use of the film. In addition, for example, the chromate treatment does not yet provide sufficient corrosion resistance to sodium chloride.
In the related art, some studies on chromate films containing an organic compound as a main component, which may substitute the above-mentioned inorganic film, have been made. The chromate films containing an organic compound as a main component are promising as chrome-free films since they have excellent formability in addition to corrosion resistance. Among them, a film having high strength and containing a urethane resin, which has excellent adhesion, as main component is particularly promising as a chromate-free film.
At present, some techniques based on a urethane resin film have been proposed. For example, Patent Document 4 discloses a technique for preparing a film containing a composite material or a mixture of a urethane resin and silicon dioxide as a main component, as an example of a film having excellent film adhesion. Patent Document 5 discloses a technique for preparing a resin film containing colloidal silica or a silane coupling agent and certain metal phosphate in a urethane resin, as an example of a film having excellent electrodepositability and weldability, and Patent Document 6 discloses a technique using a treatment agent which is a mixture of a metal phosphate, an aqueous urethane resin and oxycarboxylic acid compound.
However, as a range of usage of the above-mentioned films is widened, requirements for the films become stricter. In the techniques disclosed in Patent Documents 4 to 6, adaptability of a structure of the urethane resin is not sufficiently considered, it may be difficult to secure strict performance such as alkali resistance, workability, corrosion resistance and the like, and solvent resistance may be deteriorated since it is difficult to obtain sufficient film formability.
In the mean time, as an example for techniques having an object of providing anti-rust property, Patent Document 7 discloses a technique for preparing a film formed by adding a silane coupling agent to a hydrophilic urethane resin, and Patent Document 8 discloses a technique for preparing an anti-rust coating agent formed by mixing an aqueous silica, a silane coupling agent, a thiocarbonyl group-contained compound and phosphoric ions with a mixture of an aqueous polyurethane resin and an aqueous polyolefin resin.
However, in the techniques disclosed in Patent Documents 7 and 8, like Patent Document 4, since adaptability of a structure of the resin is not sufficiently considered, it may be difficult to secure strict performance. In addition, in the techniques disclosed in Patent Documents 7 and 8, since the silane coupling agent is added to the resin, solution reactivity may become unstable, thereby making performance irregular depending on manufacture conditions. In addition, since it is difficult for these techniques to provide sufficient crosslinking reaction stably, these techniques have a problem of deterioration of solvent resistance required to clean out dusts and the like with solvent.
In addition, Patent Document 9 discloses a technique in which a silanol group is contained in a polyurethane resin. However, in this technique, although it makes it possible to form a lubricating film having good solution stability and high elasticity which endures continuous formation by a high surface pressure process, since the film and a resin structure are not designed in consideration of corrosion resistance, solvent resistance and other characteristics, the formed film may not show sufficient anti-rust performance.
[Patent Document 1] Japanese Patent Publication No. S53-47774
[Patent Document 2] Japanese Patent Publication No. S58-31390
[Patent Document 3] Japanese Unexamined Patent Application, First Publication No. H04-293789
[Patent Document 4] Japanese Patent Publication No. H06-7950
[Patent Document 5] Japanese Patent Publication No. H06-71579
[Patent Document 6] Japanese Unexamined Patent Application, First Publication No. 2001-181855
[Patent Document 7] Japanese Unexamined Patent Application, First Publication No. 2001-59184
[Patent Document 8] Japanese Unexamined Patent Application, First Publication No. 2001-164182
[Patent Document 9] Japanese Unexamined Patent Application, First Publication No. 2001-234119